1. Field of the Invention
The present invention relates to an image forming apparatus comprising a latent image bearing member and developing means having a plurality of developing devices and provided opposite the latent image bearing member at a predetermined developing position.
2. Related Background Art
Various types of conventional color image developing apparatuses are known. A developing step common to these various types of apparatuses involves separating an original image either into three colors of yellow, magenta and cyan, or four colors, additionally including black, and forming an electrostatic latent image for each color on a latent image bearing member (e.g., a photosensitive drum 202, (as shown in FIG. 1). Each electrostatic latent image then is developed with toner by a developing device of a corresponding color. In this developing step, the developing device of each color executes the developing operation at a position adjacent to (or in contact with) the latent image bearing member. In one type of system, the developing devices of all the colors are disposed adjacent to the latent image bearing member; in another type of system, a developing device change-over portion sequentially brings the developing devices of the corresponding colors into the vicinity of (or into contact with) the latent image bearing member.
Various systems for changing over the developing devices, including a slide mounting system, a rotary drum system (also referred to as a rotary color developing system), and the like are known, with the rotary drum system being common. Referring to FIG. 1, in the rotary drum system a stepping motor (not shown) rotates a rotary color developing device 203 around a rotation shaft 200 so as to selectively bring a predetermined developing device 221 to 224 adjacent to or in contact with the latent image bearing member 202. The developing devices 221 to 224 selectively are provided according to the separated color to be developed. Therefore, compared with the configuration in which the developing devices of respective colors are disposed around the photosensitive drum, this configuration is advantageous in that it permits a reduction in size of the apparatus, and establishes a common architecture for the developing devices. The common architecture for the developing devices permits individual replacement of the developing devices as process cartridges, thus reducing toner supply problems and achieving a significant cost reduction.
However, in the rotary drum system, the rotary color developing device 203 takes time to rotate when the developing devices change over. This change-over time is greater than the processing time of the slide-mounting system. This disadvantage significantly influences, in particular, First Copy Output Time (FCOT), that is the time for outputting the first sheet of paper from the start of image formation in a rotary drum system having all four colors (yellow, magenta, cyan, black) mounted in a rotary color developing device for monochrome or color development.
For example, in the case of a rotary color developing device 203 with developing devices of black, yellow, magenta, and cyan mounted in this order, a developing operation is executed by rotating the rotary color developing device 203 around the rotation shaft 200, with a rotary stepping motor, so as to selectively bring a predetermined developing device of the color to be developed initially to a developing position adjacent to (or in contact with) the photosensitive drum 202. In the case of monochrome development, the initial color is black, and in the case of color development, it is yellow. However, it cannot be determined which of the black or yellow developing devices the rotary color developing device 203 should be switched to until it is determined whether the initial image is a monochrome image or a color image. Therefore, the electrostatic latent image formation start timing is calculated based on the developing device change-over completion scheduled time so that rotation of the rotary color developing device 203 is started after determining whether the original image is a monochrome image or a color image. Thus, the time needed for changing over the developing device delays the electrostatic latent image formation starting time. This limitation has been an obstacle for shortening the FCOT.